"The metropolitan" is not "laity". Metropolitan is (usually) the archbishop. For example, the Pope is the "Metropolitan of the Roman province".

Another point is that it is not that clear if the canon concerns election of a new bishop, or his consecration.

Anyway, assuming that it does speak about election, most likely now the Popes usually appoint bishops directly because now it is no longer impractical to have the bishops appointed by the Pope himself. At the time this canon was written, just sending the Pope a message saying that a bishop has to be appointed would have taken a lot of time.